[3.13] gh-122765: make prompt in activate.csh robust against unbalanced quotes and newlines (GH-123751) (GH-124185)

gh-122765: make prompt in activate.csh robust against unbalanced quotes and newlines (GH-123751)
(cherry picked from commit a15a584bf3f94ea11ab9363548c8872251364000)

Co-authored-by: Jacek <jacek.duszenko@gmail.com>
Co-authored-by: Brett Cannon <brett@python.org>